Required characteristics of steel pipes (line pipes) used as natural gas pipelines have become strict, and the demand of steel pipes with a high toughness in ultracold temperatures has been increased so as to withstand a cryogenic condition (≦−45° C.).
Steel pipes have been frequently used as line pipes; however, the steel pipes have been hardly actually used to meet the demand of the high toughness at the ultracold temperatures because the quality of a seam (electric resistance welded portion) is not enough. Very small oxide may be generated during electric resistance welding and remain within the seam (welded portion) even after the welding is completed. This may decrease an absorbed energy of a steel pipe when a Charpy impact test is carried out at an ultracold temperature. Hence, a necessary toughness is not stably obtained.
Conventionally, various trials have been made to decrease the remaining very small oxide. A typical technique is sealed welding in which welding equipment and a portion of a steel pipe near a welded portion are enclosed, and electric resistance welding is carried out while oxygen concentration is lowered by way of inert gas or the like (for example, see Japanese Unexamined Patent Application Publication No. 4-178281).
The above-mentioned technique, sealed welding, in which the welding equipment and the periphery thereof are enclosed to be sealed from the ambient air and filled with the inert gas instead, is widely practically used in a mill for manufacturing a small-diameter pipe with an outer diameter of φ165 mm or smaller. As the outer diameter is increased, the welding equipment is increased in size, and hence a sealed region has to be increased. It is industrially difficult to be completely sealed. Thus, sealed welding is hardly practically used in a mill for manufacturing a middle-diameter pipe or a large-diameter pipe with an outer diameter larger than φ165 mm.
In light of this, we have suggested a method for manufacturing an electric resistance welded steel pipe in Japanese Unexamined Patent Application Publication No. 2007-160383 (application date of which is later than the priority date of the subject application). The method can provide an electric resistance welded steel pipe with a high toughness even at ultracold temperatures without sealed welding.
In particular, the electric resistance welded steel pipe is manufactured such that a steel strip cut into a predetermined width is continuously shaped by roll forming to obtain a substantially tubular open pipe, and both edges of the open pipe are welded by electric resistance welding. More specifically, high-frequency current is applied to the edges of the open pipe, thereby generating Joule heat, the edges are heated and melted with the Joule heat, and then the edges are joined and pressure-welded.
At this time, during conventional electric resistance welding, edges of the open pipe have a merely rectangular shape as shown in FIG. 8A. Current may be concentrated at positions near outer and inner surfaces of the edges, whereas the current density is low at a center portion in a strip-thickness direction. Unevenness of the current density in the strip-thickness direction may directly result in temperature distribution being uneven. Further, since the temperature at the center portion in the strip-thickness direction is low, this phenomenon may be an essential factor that leads a discharge failure of oxide.
Thus, referring to a cross-sectional shape shown in FIG. 8B, Japanese Unexamined Patent Application Publication No. 2007-160383 provides a configuration in which a predetermined tapered shape (groove shape) is applied to corner portions at the outer and inner surfaces of the edges of the open pipe, to address the unevenness in the current density in the strip-thickness direction, to efficiently discharge the oxide from the welded portion, and to provide the electric resistance welded steel pipe with a high toughness at ultracold temperatures.
In an actual operation, however, the groove shapes of the edges may not achieve the predetermined shape because of a dimensional variation such as a camber (curve), or strength unevenness, of a steel strip serving as a base material. In such a case, the temperature distribution may vary at the welded portion, the electric resistance welded steel pipe with a desirable toughness may not be stably manufactured.
It could therefore be helpful to provide a method for manufacturing an electric resistance welded steel pipe having a good toughness at a welded portion, the method being capable of stably manufacturing an electric resistance welded steel pipe having a desirable toughness at a welded portion although a steel strip serving as a base material has a dimensional variation.